It is described in the U.S. Pat. No. 5,447,958 that the compounds of the above formula (1) have excellent therapeutic effects against hypertension, congestive heart failure, angina pectoris or prostatic hypertrophy. In addition, in the above patent, a process for preparing the compounds of the formula (1) by reacting hydrochlorides of the compounds of the following formula:                 with the compounds of the following formula:         is disclosed. However, the above process has many drawbacks such as an extremely low yield (about 45%), a high production costs due to the purification by column chromatography and inapplicability in a large-scale production.        